This invention relates to an improved heat pipe and particularly, to one having separate flow circuits for the vapor and liquid phases of the working fluid and means for distributing the returned liquid throughout the heat pipe evaporator.
Heat pipes are devices which efficiently transfer heat from their evaporator section to their condenser section. Working fluid inside the heat pipe absorbs heat in its evaporator portion causing the working fluid to vaporize. The vapor is transferred to the heat pipe condenser where it condenses, thus giving up its latent heat of evaporation. Liquid sodium and numerous other working fluids are used for heat pipes, depending on the temperature and pressure ranges of operation. Typically, the evaporator and condenser portions of the heat pipe are separated and the vapor and liquid working fluids flow within a connecting transport tube. As a means of distributing the liquid working fluid over the internal surface of the evaporator, a porous wick in the form of a woven mesh is often used which lines the inside surfaces of the heat pipe. The wick, due to the high capillary pressure it provides, causes returned liquid working fluid to be distributed about the surfaces of the evaporator.
Some heat pipe designs have a finned evaporator for absorbing heat from hot gases generated by a combustion furnace, internal combustion engine, or other sources. Heat transferred to the heat pipe condenser is dissipated to the environment or converted into another form of energy. In one system of the above type, the evaporator absorbs heat from hot flue gases from a combustor and the vaporized working fluid powers a Stirling cycle engine which provides a rotary or reciprocating output which can be employed to generate electricity, do direct work, etc.
In the application mentioned above in which a finned evaporator is used in connection with a Stirling engine or other applications where high fluid flow rates occur, a number of design constraints are presented. Since the vaporized working fluid leaving the evaporator being transmitted to the condenser or Stirling engine flows in a direction opposite that of liquid flow being returned to the evaporator, a problem of liquid entrainment within the vapor is presented. Such entrainment can prevent liquid from being returned to the evaporator resulting in drying out of the evaporator and possible perforation of the heat pipe housing caused by overheating.
For heat pipes with a finned evaporator, it is difficult to evenly distribute the returned liquid working fluid among all of the fins of the evaporator. Due to the finned evaporator configuration, the flow resistance of the liquid returned to a single point in the evaporator to the remotely located fins would be excessive to efficiently transport the liquid to those areas. Many heat pipe applications require the device to operate in tipped orientations. Therefore, any systems for distributing working fluid about the evaporator should be capable of operating through a range of heat pipe inclinations.
In the design of a heat pipe system of the type previously described, it is desirable to provide an excess of working fluid in order to accommodate a range of heat transfer rates of the heat pipe system. Excess amounts of liquid which are not being used for heat transfer must be stored. Simply allowing excess liquid to collect in the evaporator fins is unacceptable since the problems of boiling and shock waves would be encountered in those areas. Accordingly, there is a need to provide a system for storing liquid working fluid remote from the evaporator fins.